Refs #23919 -- Stopped inheriting from object to define new style classes.

Tests and docs complement to cecc079168.

docs/topics/class-based-views/generic-editing.txt

@@ -233,7 +233,7 @@ works for AJAX requests as well as 'normal' form POSTs::
     from django.views.generic.edit import CreateView
     from myapp.models import Author
 
-    class AjaxableResponseMixin(object):
+    class AjaxableResponseMixin:
         """
         Mixin to add AJAX support to a form.
         Must be used with an object-based FormView (e.g. CreateView)

docs/topics/class-based-views/mixins.txt

@@ -603,7 +603,7 @@ For example, a simple JSON mixin might look something like this::
 
     from django.http import JsonResponse
 
-    class JSONResponseMixin(object):
+    class JSONResponseMixin:
         """
         A mixin that can be used to render a JSON response.
         """